Vehicle with braking force retaining unit

ABSTRACT

A vehicle with a braking force retaining unit, which retains braking force after releasing the brake pedal, wherein driving force of the vehicle is increased if displacement of the vehicle is detected while the braking force retaining unit is in operation.

FIELD OF THE INVENTION

The present invention relates to a vehicle having a braking forceretaining unit, which is capable of retaining a braking force afterreleasing the brake pedal such that the braking force continuously actson the vehicle.

BACKGROUND OF THE INVENTION

Braking force retaining units are known for continuously retainingbraking force on a vehicle after releasing the brake pedal such that thebraking force continuously acts on the vehicle. Such a braking forceretaining unit facilitates a smooth starting operation of the vehicle ona slope without unintentional backward displacement of the vehicle.

For example, Japanese Patent Publication No.9-202159 discloses a brakingforce control unit utilizing a traction control system. Such a brakingforce control unit retains a certain level of braking force by thecontrol of the traction control system until driving force detectingmeans detects strong driving force (when starting driving force occurs)switched from weak driving force. When driving force is switched to thestrong driving force, the retained braking force is released, therebypreventing unintentional backward displacement of the vehicle on an upslope.

The applicant also discloses a brake fluid pressure retaining unitutilizing a brake fluid pressure reduction speed control means inJapanese Patent Application No.10-370249. The brake fluid pressureretaining unit retains braking force until starting driving force isexerted on the vehicle in such a way that reduction speed of brake fluidpressure within a wheel cylinder is less than that of brake pedal loadapplied by a driver and the brake fluid pressure is gradually reduced.When starting driving force is exerted to start the vehicle, the brakefluid pressure retaining unit releases brake fluid pressure within thewheel cylinder so as to release braking force. Therefore, the driver canstart the vehicle on an up slope without unintentional backwarddisplacement. On a down slope, the driver can start the vehicle byreleasing or partly loosening the brake pedal.

However, on a steep slope, even if the vehicle is equipped with theabove braking force retaining unit, the vehicle often displacesbackwards with the displacement force derived from the vehicle's ownweight being greater than the retained braking force. This can happenwhile the braking force retaining unit is in operation. Especially inthe aforementioned braking force control unit, retained braking forcemay be decreased to a lower value than the initial braking force valuethat is the braking force retained before starting the release of thebrake pedal. In the aforementioned brake fluid pressure retaining unit,retained braking force is not held in a certain value, but is graduallydecreased by the brake fluid pressure reduction speed control means. Thevehicle tends to displace backwards on a slope if retained braking forceis decreased, for the purpose of ensuring a smooth release of thebraking force, in advance of occurrence of starting driving force.

With the foregoing drawbacks of the prior art in view, the presentinvention seeks to provide a vehicle with a braking force retainingunit, which quickly restricts backward displacement of the vehicle whenthe vehicle starts to displace backward on a slope.

SUMMARY OF THE INVENTION

According to the present invention, there is provided a vehicle with abraking force retaining unit, which retains braking force afterreleasing the brake pedal, wherein the vehicle further comprises meansfor increasing driving force if displacement of the vehicle is detectedwhile the braking force retaining unit is in operation.

Another object of the invention is to provided a vehicle with a brakingforce retaining unit, which retains braking force after releasing thebrake pedal, wherein driving force of the vehicle is increased ifdisplacement of the vehicle is detected while the braking forceretaining unit is in operation.

In such a vehicle with the braking force retaining unit, backwarddisplacement of the vehicle is restricted, even if displacement of thevehicle occurs with the displacement force derived from the vehicle'sown weight being greater than the retained braking force, since drivingforce is immediately increased when detecting backward displacement(displacement) of the vehicle. Increasing driving force of the vehicleincludes the following two cases, these are: (1) driving force occursfrom zero and increases, and (2) already existing driving force isfurther increased.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG.1 shows a system configuration of a vehicle with a braking forceretaining unit according to the invention.

FIG. 2 shows construction of the braking force retaining unit of FIG. 1.

FIG. 3 shows the control logic of the braking force retaining unit ofFIG. 2, in which FIG. 3A shows the control logic for retaining brakefluid pressure, and FIG. 3B shows the control logic for permitting anoperation of the braking force retaining unit.

FIG. 4 shows control of a driving force control unit according to apreferred embodiment of the invention, in which FIG. 4A shows thecontrol logic for switching to a weak creep condition, FIG. 4B shows thecontrol logic for switching to a strong creep condition for driving, andFIG. 4C shows the control logic for switching to a middle creepcondition.

FIG. 5 shows the control logic of a driving motor stopping unitaccording to a preferred embodiment of the invention, in which theengine is automatically turned off.

FIG. 6 shows control of the braking force retaining unit, in which FIG.6A shows the control logic for releasing brake fluid pressure to beretained, and FIG. 6B shows the control logic for judging a creep risingcondition.

FIG. 7 shows control of the driving force control unit, in which FIGS.7A and 7B show the control logic for switching to the strong creepcondition. Here, FIG. 7A shows a backward displacement detecting versionof the vehicle, and FIG. 7B shows a vehicle movement detecting version.

FIG. 8 shows control of the driving motor stopping unit, in which FIGS.8A and 8B show the control logic for automatically turning on theengine. Here, FIG. 8A shows a backward displacement detecting version ofthe vehicle, and FIG. 8B shows a vehicle movement detecting version.

FIG. 9 shows a way of detecting backward displacement of the vehicle, inwhich FIG. 9A shows a construction thereof, FIG. 9B shows a pulse phasefor {circle around (1)} direction of FIG. 9A, and FIG. 9C shows a pulsephase for {circle around (2)} direction of FIG. 9A.

FIG. 10 is a time chart for the control of the vehicle with the brakingforce retaining unit, in which the engine is automatically turned off.Here, (a) indicates relations (increase or decrease) between drivingforce and braking force, and (b) indicates conditions (ON/OFF) ofsolenoid valves.

FIG. 11 is a time chart for the control of the vehicle with the brakingforce retaining unit, in which the engine is not automatically turnedoff. Here, (a) indicates relations (increase or decrease) betweendriving force and braking force, and (b) indicates conditions (ON/OFF)of solenoid valves.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A vehicle with a braking force retaining unit according to the presentinvention (hereinafter referred to as a vehicle) includes a brakingforce retaining unit, which continuously retains braking force afterreleasing the brake pedal. The vehicle is further equipped with adriving force control unit or/and a driving motor stopping unit. Thedriving force control unit decreases creep driving force when the motoris in idling condition and the brake pedal is depressed under or equalto a certain vehicle speed. The driving motor stopping unit canautomatically turn off the motor while the vehicle stops. Further, thedriving force control unit increases the decreased driving force, whichhas been reduced by the driving force control unit or completely lostwith the automatic motor stop operation of the driving force controlunit. The vehicle shown in this preferred embodiment is a hybrid typevehicle having an engine as an internal combustion engine operable bygasoline and the like, and an electric motor as a driving motor, and isprovided with a belt-type continuously variable transmission(hereinafter referred to as CVT) as a transmission. In this vehicle, thedriving motor is not restricted merely to an engine and a motor.Similarly, the transmission is not restricted to a particular type. Thetransmission may be an automatic transmission having a torque converteror a manual transmission.

In creep running, the vehicle with an automatic transmission movesslowly as if it creeps on the ground when the transmission is selectedto a running range such as D range or R range and the accelerator pedalis released (the driving motor is in idling condition).

<System Configuration>

The system configuration of a vehicle according to the present inventionwill be described with reference to FIG. 1. The vehicle shown in thisembodiment includes an engine 1 and a motor 2 as a driving motor, and isequipped with CVT 3 as a transmission.

The engine 1 is controlled at a fuel injection electronic control unit(hereinafter referred to as FI ECU). The FI ECU is integrallyconstructed with a management electronic control unit (hereinafterreferred to as MG ECU), and it is incorporated in a fuelinjection/management electronic control unit 4 (hereinafter referred toas FI/MG ECU). The motor 2 is controlled at a motor electronic controlunit 5 (hereinafter referred to as MOT ECU). Further, the CVT 3 iscontrolled at a CVT electronic control unit 6 (hereinafter referred toas CVT ECU).

A drive axle 7 provided with driving wheels 8, 8 is mounted to the CVT3. Each driving wheel 8 is provided with a disc brake 9, which includesa wheel cylinder WC and the like (FIG. 2). The wheel cylinders WC of thedisc brakes 9,9 are connected to a master cylinder MC through a brakingforce retaining unit RU. When the driver depresses the brake pedal BP,brake pedal load generated is transmitted to the master cylinder MCthrough the master power MP. The brake switch BSW detects whether thebrake pedal BP is depressed or not.

The engine 1 is an internal combustion engine, which makes use ofthermal energy. The engine 1 drives the driving wheels 8, 8 through theCVT 3 and the drive axle 7. In order to improve fuel consumption, theengine 1 may be automatically turned off while the vehicle stops. Forthis reason, the vehicle is provided with a driving motor stopping unitfor automatically turning off the engine 1 when a certain automaticengine stop condition is satisfied.

The motor 2 has an assist mode for the assist of the engine drive withthe use of electric energy from a non-shown battery. The motor 2 has aregeneration mode for converting the kinetic energy derived from therotation of the drive axle 7 into electric energy. When the engine doesnot require the assist from the assist mode (such as for starting on adown slope or deceleration of the vehicle), the thus converted electricenergy is stored in a non-shown battery. Further, the motor 2 has anactuation mode for actuating the engine 1.

The CVT 3 includes an endless belt winded between a drive pulley and adriven pulley so as to enable continuously variable gear ratio bychanging a winding radius of the endless belt. Change of the windingradius is achieved by changing each pulley width. The CVT 3 engages astarting clutch and an output shaft so as to transmit the output of theengine 1 converted by the endless belt into the drive axle 7 throughgears at the output side of the starting clutch. The vehicle equippedwith the CVT 3 enables creep running, and such a vehicle requires adriving force control unit DCU for changing driving force to be utilizedfor the creep running.

Creep driving force is adjusted by the engaging force of the startingclutch (driving force transmission capacity). The creep driving forceincludes three conditions, i.e., a strong condition, weak condition, anda middle condition between the strong and weak conditions. In thisembodiment, the strong condition is referred to as a strong creepcondition, and the weak condition is referred to as a weak creepcondition, and further the middle condition is referred to as a middlecreep condition. Further, the strong creep condition includes twodriving force levels, i.e., a strong level and a weak level. The stronglevel is merely referred to as a strong creep condition, and the weaklevel is referred to as a strong creep condition for driving. In thestrong creep condition, driving force is adjusted so as to keep thevehicle stationary on a slope having an inclination angle of 5 degrees.In the strong creep condition, vehicle speed is under or equal to 5km/h. In the strong creep condition for driving, driving force isadjusted to be less than that in the strong creep condition. The strongcreep condition for driving is a preliminary condition before switchingto the weak creep condition, in which vehicle speed is over 5 km/h. Inthe weak creep condition, almost no driving force is obtained. In themiddle creep condition, driving force is controlled substantially to ahalf extent between the strong creep condition and the weak creepcondition. The middle creep condition is an intermediate condition whendriving force is stepwise decreased in the process of switching from thestrong creep condition to the weak creep condition. The strong creepcondition is achieved when the accelerator pedal is released (idlingcondition) and the positioning switch PSW selects a running range (Drange, L range or R range) and further the brake pedal BP is released.In the strong creep condition, the vehicle moves slowly as if it creepson the ground. Meanwhile, the vehicle stops or moves at an extremely lowspeed in the weak creep condition, with the brake pedal BP depressedunder or equal to a certain vehicle speed.

Range positions of the positioning switch PSW are selected by a shiftlever. Such range positions are selected from P range to be used forparking the vehicle, N range as a neutral range, R range for backwardrunning, D range to be used for a normal run, and L range to be used forobtaining a sudden acceleration or strong engine brake. The term“running range” indicates a range position, at which the vehicle canmove. In this vehicle, the running range includes D range, L range and Rrange. Further, when the positioning switch PSW selects D range, D modeas a normal running mode and S mode as a sports mode can be selected bya mode switch MSW. Information of the positioning switch PSW and themode switch MSW is transmitted to the CVT ECU 6 and further to a meter10. The meter 10 indicates the range information and the modeinformation selected by the positioning switch PSW and the mode switch,respectively.

FI ECU contained in the FI/MG ECU 4 controls the amount of fuelinjection so as to achieve the optimum air fuel ratio, and it alsogenerally controls the engine 1. Various kinds of information such as athrottle angle and conditions of the engine 1 is transmitted to the FIECU such that the engine 1 is controlled based on such information. TheMG ECU contained in the FI/MG ECU 4 mainly controls the MOT ECU 5 aswell as judges automatic engine stop conditions and automatic engineactuation conditions. The MG ECU receives information as to conditionsof the motor 2 and other information such as conditions of the engine 1from the FI ECU, and based on such information it sends instructionsabout mode switching of the motor 2 to the MOT ECU 5. Further, the MGECU receives information such as conditions of the CVT 3, conditions ofthe engine 1, range information of the positioning switch PSW,conditions of the motor 2 and the like, and based on such information itjudges whether the engine 1 should be automatically stopped orautomatically actuated.

The MOT ECU 5 controls the motor 2 based on a control signal from theFI/MG ECU 4. The control signal from the FI/MG ECU 4 includes modeinformation instructing actuation of the engine 1 by the motor 2,assistance of the engine actuation or regeneration of electric energy,and an output required value to the motor 2, and the MOT ECU 5 sends anorder to the motor 2 based on such information. Further, the MOT ECU 5receives information from the motor 2 and transmits information such asthe amount of generated energy and the capacity of the battery to theFI/MG ECU 4.

The CVT ECU 6 controls the transmission gear ratio of the CVT 3, theengaging force of the starting clutch (the driving force transmissioncapacity) and the like. Various kinds of information such as conditionsof the CVT 3, conditions of the engine 1, range information of thepositioning switch PSW and the like is transmitted to the CVT ECU 6, andbased on such information the CVT ECU 6 transmits a signal to the CVT 3,the signal of which includes control of hydraulic pressure of eachcylinder provided at the drive pulley and the driven pulley of the CVT3, and control of hydraulic pressure of the starting clutch. As shown inFIG. 2, the CVT ECU 6 comprises a control unit CU for the ON/OFF control(shut-off/communicate) of solenoid valves SV(A), SV(B) of the brakingforce retaining unit RU. The CVT ECU 6 transmits a signal for ON and OFFthe solenoid valves SV(A), SV(B) to the braking force retaining unit RU.The CVT ECU 6 judges switching of creep driving force as well as judgesincrement of driving force upon detecting a movement (or a backwarddisplacement) of the vehicle while the braking force retaining unit RUis in operation. The CVT ECU 6 transmits the judgement information tothe driving force control unit DCU of the CVT 3. The CVT ECU 6 furthercomprises a failure-detecting unit DU for the purpose of detectingmalfunction of the braking force retaining unit RU.

The disk brakes 9, 9 are constructed such that a disk rotor rotatablewith the driving wheel 8 is pressed between the brake pads moved by thewheel cylinder WC (FIG. 2) and braking force is obtained by thefrictional force therebetween. Brake fluid pressure within the mastercylinder MC is transmitted to the wheel cylinders WC through the brakingforce retaining unit RU.

The braking force retaining unit RU continuously retains brake fluidpressure within a wheel cylinder WC, i.e. braking force, afterdepression of the brake pedal BP is released. The braking forceretaining unit RU comprises a control unit CU within the CVT ECU 6.Construction of the braking force retaining unit RU will be describedlater in greater detail with reference to FIG. 2.

ON/OFF operation of the solenoid valve is meant as follows: In thenormally open type solenoid valve, when the solenoid valve is ON, thesolenoid valve closes and is in closed condition, and when the solenoidvalve is OFF, the solenoid valve opens and is in open condition.Meanwhile, in the normally closed type solenoid valve, when the solenoidvalve is ON, the solenoid valve opens and is in open condition, and whenthe solenoid valve is OFF the solenoid valve closes and is in closedcondition. The solenoid valves SV(A), SV(B) in the preferred embodimentare of normally open type. A driving circuit within the control unit CUcarries out or ceases a supply of electric currents to respective coilsof the solenoid valves SV(A), SV(B) so as to ON and OFF the solenoidvalves SV(A), SV(B).

A master cylinder MC is a device for converting a brake pedal depressioninto hydraulic pressure. In order to assist the brake pedal depression,a master power MP is provided between the master cylinder MC and thebrake pedal BP. The master power MP enhances braking force by way ofapplying negative pressure of the engine 1 or compressed air to thedriver's brake pedal depression force. A brake switch BSW is provided atthe brake pedal BP so as to detect whether or not the brake pedal BP isdepressed.

The driving force control unit DCU of the vehicle is incorporated in theCVT 3. The driving force control unit DCU variably controls the drivingforce transmission capacity of the starting clutch, thereby changingcreep driving force. The driving force control unit DCU increasesdriving force when displacement of the vehicle (or backward displacementof the vehicle) is detected. The driving force control unit DCUcomprises CVT ECU 6, which judges switching of creep driving force aswell as judges the increment of driving force upon detecting a movement(or a backward displacement) of the vehicle, and which transmits, basedon the judgement information, a hydraulic pressure command value to alinear solenoid valve controlling the engagement hydraulic pressure ofthe starting clutch.

The driving force control unit DCU controls the driving forcetransmission capacity of the starting clutch and switches to the drivingforce in each creep condition when the CVT ECU 6 judges conditions(hereinafter described) required for a weak creep condition, middlecreep condition, strong creep condition or a strong creep condition fordriving. Further, the driving force control unit DCU increases thedriving force transmission capacity of the starting clutch and switchesto the strong creep condition if a movement or backward displacement ofthe vehicle is detected. Detection for backward displacement or movementof the vehicle is included in the condition required for a strong creeporder or the condition required for an automatic engine actuation order.The CVT ECU 6 judges the above conditions, and it transmits a hydraulicpressure command value to a linear solenoid valve of the CVT 3, wherethe engagement hydraulic pressure of the starting clutch is controlled.In the driving force control unit DCU, the driving force transmissioncapacity (engagement force) of the starting clutch is adjusted at theCVT 3 based on the hydraulic pressure command value, and therebyswitching the creep driving force. Since the driving force control unitDCU decreases driving force, deteriorated fuel consumption of thevehicle is prevented. The term “driving force transmission capacity”indicates the maximum driving force (driving torque) transmitted by thestarting clutch. When a failure-detecting unit DU detects malfunction ofthe braking force retaining unit RU, the switching operation of thedriving force control unit DCU to the weak creep condition isrestricted.

The driving motor stopping unit incorporated in the vehicle isconstructed by the FI/MG ECU 4 and others. The driving motor stoppingunit enables an automatic engine stop operation while the vehicle stops.The automatic engine stop conditions are judged at the FI/MG ECU 4 andthe CVT ECU 6. The automatic engine stop conditions will be describedlater. When all of the automatic engine stop conditions are satisfied,the FI/MG ECU 4 sends an engine stop order to the engine 1 so as toautomatically turn off the engine 1. Since the driving motor stoppingunit automatically turns off the engine 1, improved fuel consumption ofthe vehicle is achieved.

The FI/MG ECU 4 and the CVT ECU 6 judges automatic engine actuationconditions while the driving motor stopping unit automatically turns offthe engine 1. When all of the automatic engine actuation conditions aresatisfied, the FI/MG ECU 4 sends an engine actuation order to the MOTECU 5. The MOT ECU 5 further transmits an engine actuation order to themotor 2. The motor 2 then automatically actuates the engine 1, and atthe same time driving force is switched to the strong creep condition.The automatic engine actuation conditions will be described later.

Further, when the failure-detecting unit DU detects malfunction of thebraking force retaining unit RU, operation of the driving motor stoppingunit is prohibited.

Signals to be transmitted and received in this system will be described.With reference to FIG. 1, the letter “F_” in front of each signalindicates that the signal is flag information, which is either 0 or 1.The letter “V_” indicates that the signal is numerical information (unitis optional), and the letter “I_” indicates that the signal includesplural kinds of information.

A signal transmitted from the FI/MG ECU 4 to the CVT ECU 6 will bedescribed. V_MOTTRQ represents an output torque value of the motor 2.F_MGSTB is a flag showing whether all of the engine stop conditionsjudged at the FI/MG ECU 4 are satisfied. If all the conditions aresatisfied, the numeral 1 is given, and if not, the numeral 0 is given.The automatic engine stop conditions regarding the F_MGSTB will bedescribed later. When the F_MGSTB and F_CVTOK (hereinafter described)are both turned to 1, the engine 1 is automatically turned off. When oneof these flags is turned to 0, the engine 1 is automatically turned on.

A signal transmitted from the FI/MG ECU 4 to the CVT ECU 6 and the MOTECU 5 will be described. V_NEP represents engine speed.

A signal transmitted from the CVT ECU 6 to the FI/MG ECU 4 will bedescribed. F_CVTOK is a flag showing whether all of the engine stopconditions judged at the CVT ECU 6 are satisfied. If all the conditionsare satisfied, the numeral 1 is given, and if not, the numeral 0 isgiven. The automatic engine stop conditions regarding the F_CVTOK willbe described later. F_CVTTO is a flag showing whether the oiltemperature of the CVT 3 is over a certain value. If the oil temperatureis at the certain value or more, the numeral 1 is given, and if the oiltemperature is below the value, the numeral 0 is given. The oiltemperature of the CVT 3 is obtained from an electrical resistance valueof the linear solenoid valve controlling hydraulic pressure of thestarting clutch at the CVT 3. F_POSR is a flag showing a conditionwhether the positioning switch PSW is selected in R range. If thepositioning switch PSW selects R range, the numeral 1 is given, and ifnot, the numeral 0 is given. F_POSDD is a flag showing a conditionwhether the positioning switch PSW selects D range and the mode switchMSW selects D mode. If D range and D mode (D range/D mode) are selected,the numeral 1 is given, and if not, the numeral 0 is given. When theFI/MG ECU 4 does not receive any information indicating D range/D mode,R range, P range or N range, the FI/MG ECU 4 judges that either Drange/s mode or L range is selected.

F_MCRPON is a flag whether or not driving force is in the middle creepcondition. The numeral 1 is given in the middle creep condition, and ifnot, the numeral 0 is given. When F_MCRPON is 1, the engine 1 isrequired to blow middle air in the middle creep condition (weaker airthan that in the strong creep condition). F_AIRSCRP is a strong airdemand flag in the strong creep condition. If strong air is required inthe strong creep condition, the numeral 1 is given, and if not, thenumeral 0 is given. When both F_MCRPON and F_AIRSCRP are 0, FI/MG ECUblows weak air in the weak creep condition. In order to keep the engineidle speed at a certain level regardless of the driving force in thestrong creep condition, middle creep condition or the weak creepcondition, output of the engine should be adjusted by way of blowingcorresponding air to the strong creep condition, middle creep conditionor the weak creep condition. When the driving force is in the strongcreep condition and a load of the engine 1 is higher, a strong air blow(strong air in the strong creep condition) is required. The term “airblow” means the supply of air from an air passage by-passing a throttlevalve of the engine 1 to an intake pipe positioned at a downstream ofthe throttle valve. Strength (amount) of air to be blown is adjusted bycontrolling degrees of opening of the air passage.

A signal transmitted from the engine 1 to the FI/MG ECU 4 and the CVTECU 6 will be described. V_ANP represents a negative pressure value atthe intake pipe of the engine 1. V_TH represents a throttle angle. V_TWrepresents a temperature of the cooling water at the engine 1. V_TArepresents the intake temperature of the engine 1. The brake fluidtemperature in the braking force retaining unit RU disposed within theengine compartment is obtained from the intake temperature. This isbecause both temperatures change with respect to the temperature at theengine compartment.

A signal transmitted from the CVT 3 to the FI/MG ECU 4 and the CVT ECU 6will be described. V_VSP1 represents a vehicle speed pulse from one oftwo vehicle speed pickups provided in the CVT 3. Vehicle speed iscalculated based on this vehicle speed pulse.

A signal transmitted from the CVT 3 to the CVT ECU 6 will be described.V_NDRP represents a pulse showing the number of revolutions of the drivepulley provided at the CVT 3. V_NDNP represents a pulse showing thenumber of revolutions of the driven pulley provided at the CVT 3. V_VSP2represents a vehicle speed pulse from the other vehicle speed pickup atthe CVT 3. The V_VSP2 is more accurate than the V_VSP1, and the V_VSP2is used for calculating the amount of clutch slipping at the CVT 3.

A signal transmitted from the MOT ECU 5 to the FI/MG ECU 4 will bedescribed. V_QBAT represents a remaining capacity of the battery.V_ACTTRQ represents an output torque value of the motor 2, which is thesame as the V_MOTTRQ. I_MOT represents information such as the amount ofgenerated energy of the motor 2 showing electric loading. The motor 2generates all the electric power consumed for the vehicle including theelectric power for driving the motor.

A signal transmitted from the FI/MG ECU 4 to the MOT ECU 5 will bedescribed. V_CMDPWR represents an output required value to the motor 2.V_ENGTRQ represents an output torque value of the engine 1. I_MGrepresents information such as an actuation mode, assist mode and aregeneration mode with respect to the motor 2.

A signal transmitted from the master power MP to the FI/MG ECU 4 will bedescribed. V_M/PNP represents a negative pressure detected value at aconstant pressure chamber of the master power MP.

A signal transmitted from the positioning switch PSW to the FI/MG ECU 4will be described. N or P is transmitted as positioning information whenthe positioning switch PSW selects either N range or P range.

A signal transmitted from the CVT ECU 6 to the CVT 3 will be described.V_DRHP represents a hydraulic pressure command value transmitted to thelinear solenoid valve, which controls hydraulic pressure within thecylinder of the drive pulley at the CVT 3. V_DNHP represents a hydraulicpressure command value transmitted to the linear solenoid valve, whichcontrols hydraulic pressure within the cylinder of the driven pulley atthe CVT 3. The transmission gear ratio of the CVT 3 is changed by V_DRHPand V_DNHP. V_SCHP represents a hydraulic pressure command valuetransmitted to the linear solenoid valve, which controls hydraulicpressure of the starting clutch at the CVT 3. The engaging force of thestarting clutch (driving force transmission capacity) is changed byV_SCHP.

A signal transmitted from the CVT ECU 6 to the braking force retainingunit RU will be described. F_SOLA is a flag for ON/OFF (close/open) thesolenoid valve SV(A) of the braking force retaining unit RU (shown inFIG. 2). The numeral 1 is given for closing (ON) the solenoid valveSV(A), and the numeral 0 is given for opening (OFF) the solenoid valveSV(A). F_SOLB is a flag for ON/OFF (close/open) the solenoid valve SV(B)of the braking force retaining unit RU (shown in FIG. 2). The numeral 1is given for closing (ON) the solenoid valve SV(B), and the numeral 0 isgiven for opening (OFF) the solenoid valve SV(B).

A signal transmitted from the positioning switch PSW to the CVT ECU 6will be described. The positioning switch PSW selects N range, P range,R range, D range or L range, and the selected range is transmitted aspositioning information.

A signal transmitted from the mode switch MSW to the CVT ECU 6 will bedescribed. The mode switch MSW selects either D mode (normal runningmode) or S mode (sports running mode), and the selected mode istransmitted as mode information. The mode switch MSW is a mode selectionswitch, which works when the positioning switch PSW is set in D range.

A signal transmitted from the brake switch BSW to the FI/MG ECU 4 andthe CVT ECU 6 will be described. F_BKSW is a flag showing a conditionwhether the brake pedal BP is depressed (ON) or released (OFF). If thebrake pedal BP is depressed, the numeral 1 is given, and if the brakepedal is released, the numeral 0 is given.

A signal transmitted from the CVT ECU 6 to the meter 10 will bedescribed. The positioning switch PSW selects N range, P range, R range,D range or L range, and the selected range is transmitted as positioninginformation. Further, the mode switch MSW selects either D mode (normalrunning mode) or S mode (sports running mode), and the selected mode istransmitted as mode information.

<Construction of Braking Force Retaining Unit>

The braking force retaining unit RU mounted on the vehicle isincorporated in the brake fluid pressure circuit of the hydraulicallyoperable braking device, and is constructed by a brake fluid pressurereduction speed control means for retaining reduction speed of brakefluid pressure within the wheel cylinder less than that of brake pedalload applied by the driver.

With reference to FIG. 2, the braking force retaining unit RU and thehydraulically operable braking device BK will be described.

[Hydraulically Operable Braking Device]

With reference to FIG. 2, the hydraulically operable braking device BKof the vehicle will be described. The brake fluid pressure circuit BC ofthe hydraulically operable braking device BK comprises brake fluidpiping FP connecting the braking device BK to the master cylinder MC andthe wheel cylinders WC. Since brake is a very important factor for asafety run, the braking device BK has two separate systems of brakefluid pressure circuits BC(A), BC(B). Therefore, if one system is out oforder, the remaining system works for obtaining a minimum braking force.

Master cylinder pistons MCP, MCP are inserted into a main body of themaster cylinder MC. When the driver applies a load to the brake pedalBP, the pistons MCP, MCP are pressed and pressure is applied to brakefluid so that mechanical force is converted into brake fluid pressure,i.e., the pressure applied to the brake fluid. When the driver releasesthe brake pedal BP for removing the applied load, the pistons MCP, MCPare returned to the original position by the resilient action of returnsprings MCS, MCS and the brake fluid pressure is released at the sametime. In view of fail-safe mechanism, there are provided two separatebrake fluid pressure circuits BC. For this reason, the master cylinderMC shown in FIG. 2 is a tandem master cylinder, where two pistons MCP,MCP are connected in series so that the main body of the master cylinderMC is divided into two portions.

A master power MP (brake booster) is provided between the brake pedal BPand the master cylinder MC so as to ease the braking effort of thedriver. The master power MP shown in FIG. 2 is a vacuum servo type. Themaster power MP takes out negative pressure from an intake manifold ofthe engine 1 (not shown) so as to facilitate the braking operation ofthe driver.

The brake fluid piping FP connects the master cylinder MC and the wheelcylinders WC. The brake fluid piping FP functions as a fluid channel forbrake fluid. Brake fluid pressure generated at the master cylinder MC istransmitted to the wheel cylinders WC since a flow of the brake fluidtravels through the brake fluid piping FP. When the brake fluid pressurewithin the wheel cylinder WC is greater, the brake fluid is transmittedfrom the wheel cylinders WC to the master cylinder MC through the brakefluid piping FP. Since separate brake fluid pressure circuits BC areprovided for the reason mentioned above, there are also provided twoseparate brake fluid piping systems FP. The brake fluid pressure circuitBC such as constructed by the brake fluid piping shown in FIG. 2 is anX-piping type, where one brake fluid pressure circuit BC(A) is forbraking a front right wheel and a rear left wheel, and the other brakefluid pressure circuit BC(B) is for braking a front left wheel and arear right wheel. The brake fluid pressure circuit may be a front andrear dividing piping type, where one brake fluid pressure circuit is forbraking front wheels, and the other brake fluid pressure circuit is forbraking rear wheels.

The wheel cylinder WC is provided for each wheel 8 so that brake fluidpressure generated at the master cylinder MC and transmitted to thewheel cylinder WC through the brake fluid piping FP is converted into amechanical force (braking force) for braking wheels 8. A piston (notshown) is inserted into the wheel cylinder WC so that when the piston ispressed by brake fluid pressure, it generates braking force foractuating brake pads in the case of disc brakes or brake shoes in thecase of drum brakes.

Additionally, there may be provided brake fluid pressure control valvesfor controlling brake fluid pressure within the wheel cylinders of thefront and rear wheels.

[Braking Force Retaining Unit]

With reference to FIG. 2, the braking force retaining unit RU will bedescribed. The braking force retaining unit RU comprises a brake fluidpressure reduction speed control means for retaining reduction speed ofthe brake fluid pressure within the wheel cylinder WC less than that ofbrake pedal load applied by the driver upon starting the vehicle. Thebrake fluid pressure reduction speed control means works such that thereduction speed of brake fluid pressure within the wheel cylinder WC(reduction speed of the braking force) becomes slower than that of thebrake pedal load applied by the driver upon releasing the brake pedalBP.

The brake fluid pressure reduction speed control means having the abovefunction is made by the provision of a flow resistance against a flow ofthe brake fluid in the brake fluid pressure circuit BC. It can be madeby means for restricting the movement of the brake pedal BP, in additionto the brake fluid pressure circuit BC, so as to restrict the recoveringspeed of the brake pedal BP from a brake pedal acting position to theoriginal position in such a way that the brake pedal BP slowly returnsto its original position if the driver quickly releases the brake pedalBP. The former is for restricting a flow of the brake fluid per se,while the latter is for restricting the movement of the brake pedal BP.In either case, reduction speed of the brake fluid pressure within thewheel cylinder WC will be less than that of the brake pedal load appliedby the driver. Detailed construction of the former type will bedescribed below.

The braking force retaining unit RU including the brake fluid pressurereduction speed control means within the brake fluid pressure circuit BCof the hydraulically operable braking device BK will be described. Sincethe hydraulically operable braking device BK comprises two brake fluidpressure circuits BC(A), BC(B) of the same construction, merely onebrake fluid pressure circuit BC(A) will be described. In order torestrict a flow of brake fluid per se, the brake fluid pressure circuitBC(A) is provided with a solenoid valve SV(A) and a restriction D, andif necessary a check valve CV and a relief valve RV. In the brakingforce retaining unit RU, the brake fluid pressure reduction speedcontrol means is constructed by the solenoid valve SV(A) and therestriction D.

The solenoid valve SV(A) is ON (closed)/OFF (open) by an electric signalfrom the control unit CU. The solenoid valve SV(A) shuts off a flow ofbrake fluid within the brake fluid piping FP in its ON condition (closedcondition), thereby retaining applied brake fluid pressure within thewheel cylinder WC. The solenoid valve SV(A) shown in FIG. 2 is in OFFcondition (open condition). Provision of the solenoid valve SV(A)prevents unintentional backward displacement of the vehicle uponstarting on a slope. This is because when the driver releases the brakepedal BP, brake fluid pressure is retained within the wheel cylinder WC.The term “unintentional backward displacement” means that the vehiclemoves in an opposite direction due to its own weight (potential energy),in other words, the vehicle begins to descend backwards on a slope.

The solenoid valve SV(A) may be of both normally open and normallyclosed types. However, in view of fail-safe mechanism, a normally opentype is preferable. This is because when electricity is cut off due tomalfunction, brake does not work or brake always works in a normallyclosed type solenoid valve. In the normal operation, the solenoid valveSV(A) is ON (closed condition) when the vehicle stops, and is kept inthe ON condition until the vehicle starts to move. Conditions forswitching the solenoid valve SV(A) to ON (closed condition) or to OFF(open condition) will be described later.

The restriction D always connects the master cylinder MC and the wheelcylinders WC regardless of the ON/OFF conditions of the solenoid valveSV(A). Especially when the solenoid valve SV(A) is in ON condition(closed condition) and the driver gradually or instantly releases thebrake pedal BP, the restriction D reduces brake fluid pressure withinthe wheel cylinder WC at a certain speed by gradually transferring brakefluid from the wheel cylinders WC to the master cylinder MC. Such arestriction D may be formed by the provision of a flow control valve inthe brake fluid piping FP. Alternatively, the restriction D may beformed at a part of the brake fluid piping FP by way of a flowresistance (reduced area portion of the passage, at which a part of thesection becomes narrow).

With the provision of the restriction D when the driver gradually orinstantly releases the brake pedal BP, braking force is graduallylowered so that even if the solenoid valve SV(A) is in ON condition(closed condition), brake does not work permanently. In other words,reduction speed of brake fluid pressure within the wheel cylinder WC isless than that of brake pedal load applied by the driver. Therefore,even if the solenoid valve SV is in ON condition (closed condition),braking force is reduced after a certain period of time so that thevehicle can start to move on an up slope by driving force of the drivingmotor. Meanwhile, the vehicle can start off on a down slope due to itsown weight without requiring the accelerator pedal operation of thedriver.

The restriction D does not affect braking force as long as brake fluidpressure within the master cylinder MC due to the driver's brake pedaloperation is greater than that within the wheel cylinder WC. This isbecause brake fluid flows based on a pressure difference between thewheel cylinder WC and the master cylinder MC, i.e. from one at higherbrake fluid pressure to the other at lower brake fluid pressure. Unlessthe driver releases the brake pedal BP, the brake fluid pressure withinthe wheel cylinder WC does not lower although it may increase. Therestriction D may function as a check valve so as to prevent a counterflow from the master cylinder MC to the wheel cylinder WC.

Reduction speed of brake fluid pressure within the wheel cylinder WC isdetermined so as to keep sufficient time for pedal-changing action ofthe driver from the brake pedal BP to the accelerator pedal that is thetime for obtaining sufficient driving force of the motor to start thevehicle on the slope without unintentional backward displacementthereof. Normally, the time required for the pedal-changing action andfor obtaining sufficient driving force of the motor is about 0.5seconds.

In the case that the reduction speed of the brake fluid pressure withinthe wheel cylinder WC is faster, the vehicle will move backwards on theslope before sufficient driving force of the motor will be obtainedsince braking force after releasing the brake pedal BP will beimmediately lost even if the solenoid valve SV(A) is in ON condition(closed condition). For this reason, this braking force retaining unitRU can not achieve the purpose for facilitating the starting operationon the slope. On the contrary, in the case that the reduction speed ofthe brake fluid pressure within the wheel cylinder WC is slower, thevehicle will not move backwards on the slope after releasing the brakepedal BP since the brake is working at all times. However, extra timeand driving force is required for obtaining braking force and sufficientdriving force to move the vehicle against the slope, leading todifficulty in the starting operation.

Reduction speed for reducing brake fluid pressure within the wheelcylinder WC is determined by properties of the brake fluid or shape ofthe restriction D (cross section or length of the flow pass). Therestriction D may be employed as an integral member with a solenoidvalve SV(A) and a check valve CV. In this case, the number of parts andinstall space may be reduced.

A check valve CV is provided in case of necessity. The check valve CVtransfers brake fluid pressure generated within the master cylinder MCinto the wheel cylinders WC on condition that the solenoid valve SV(A)is in ON condition (closed condition) and the driver increases brakepedal load. The check valve CV works effectively when brake fluidpressure generated within the master cylinder MC is greater than thebrake fluid pressure within the wheel cylinder WC. The check valve CVquickly increases the brake fluid pressure within the wheel cylinder WCin accordance with the increased brake pedal load.

If an arrangement is employed such that the solenoid valve SV(A) isswitched from the ON condition (closed condition) to the OFF condition(open condition) when brake fluid pressure within the master cylinder MCbecomes greater than that within the wheel cylinder WC, there is no needto provide a check valve CV since the solenoid valve SV(A) itselfresponds to the increased brake pedal load.

A relief valve RV is also provided in case of necessity. The reliefvalve RV transfers brake fluid within the wheel cylinder WC into themaster cylinder MC until brake fluid pressure within the wheel cylinderbecomes a certain pressure level on condition that the solenoid valveSV(A) is in ON condition (closed condition) and the driver gradually orinstantly releases the brake pedal BP. The relief valve RV works whenbrake fluid pressure within the wheel cylinder WC is greater than thepredetermined brake fluid pressure and brake fluid pressure within themaster cylinder MC. Therefore, even if the solenoid valve SV(A) is in ONcondition (closed condition), extra brake fluid pressure within thewheel cylinder WC beyond the necessary brake fluid pressure is quicklyreduced to a certain brake fluid pressure. This will ensure a smoothstarting operation of the vehicle on a slope even if the driverforcefully depresses the brake pedal BP more than required. Provision ofa relief valve RV eliminates a drawback that reducing brake fluidpressure within the wheel cylinder WC only through the restriction D,after releasing the brake pedal BP, requires a lot of time.

A brake switch BSW detects whether the brake pedal BP has been depressedor not. Based on the detected value, the control unit CU sendsinstructions as to ON/OFF operations of the solenoid valves SV(A),SV(B).

The braking force retaining unit RU is constructed by the solenoidvalves SV(A), SV(B) and the restriction D. Although the braking forceretaining unit RU is constructed by means for acting on brake fluidpressure as means for acting on a braking force, other means may beemployed as long as they act on the braking force. For example,arrangement of the braking force retaining unit may be such that a servovalve is used instead of the solenoid valves SV(A), SV(B) and therestriction D, or that the recovering speed of the brake pedal BP isrestricted. In the vehicle with a traction control system, which cangenerate brake fluid pressure regardless of depression of the brakepedal, control of braking force may be carried out as a function of thetraction control system. Also, in the vehicle with an antilock brakingsystem, which can control brake fluid pressure within the wheel cylinderupon depressing the brake pedal, control of brake fluid pressure withinthe wheel cylinder may be carried out as a function of the antilockbraking system.

<Basic Operation of Braking Force Retaining Unit>

With reference to FIG. 2, basic operation of the braking force retainingunit RU will be described.

(Stop/Start Operations on Up Slope)

When the vehicle is stopped on an up slope, the driver depresses thebrake pedal BP so as to prevent the vehicle from unintentional backwarddisplacement due to its own weight. With depression of the brake pedalBP, brake fluid within the master cylinder MC is pressed, and brakefluid pressure within the master cylinder MC increases. This increasedbrake fluid pressure results in a flow of brake fluid, which flows fromthe master cylinder MC to the wheel cylinders WC through the brake fluidpiping FP and the solenoid valves SV(A), SV(B) in open condition.Therefore, brake fluid pressure generated within the master cylinder MCis converted into braking force for braking wheels, and the vehicle canbe stopped on the slope.

The control unit CU judges conditions including the vehicle beingstopped, and turns the solenoid valves SV(A), SV(B) ON (closedcondition) so as to retain brake fluid pressure within the wheelcylinder WC. The control unit CU is not required to judge whether thevehicle is stopped on a slope or not.

In the case of the arrangement where check valves CV, CV are employed,even if the solenoid valves SV(A), SV(B) are in ON condition (closedcondition), braking force is increased through the check valves CV, CVwhen the driver further increases the brake pedal load.

In order to initiate a starting operation of the vehicle on the slope,the driver releases the brake pedal BP and in stead depresses thenon-shown accelerator pedal. During the operation, since the solenoidvalves SV(A), SV(B) are in ON condition (closed condition), the vehicleis prevented from unintentional backward displacement even if the driverreleases the brake pedal BP. However, brake fluid pressure within thewheel cylinders WC gradually decreases through the restrictions D, D.

Meanwhile, when the driver depresses the accelerator pedal, drivingforce of the vehicle increases. The vehicle can start on the slope whenthe driving force of the vehicle becomes greater than the total amountof the prevention force, which prevents the vehicle from advancing theslope (displacement force due to the vehicle's own weight), and thebraking force, which has been gradually reducing.

However, when the vehicle is on a steep slope, regardless of thesolenoid valves SV(A), SV(B) being in ON condition (closed condition),the vehicle may displace backwards with the displacement force derivedfrom the vehicle's own weight being greater than the retained brakingforce. Especially when the driver instantly or gradually releases thebrake pedal BP, brake fluid pressure is gradually decreased by therestriction D, thereby decreasing braking force. In other words, sincethe retained braking force is gradually decreasing, the displacementforce due to the vehicle's own weight may be greater than the retainedbraking force.

Provision of the restrictions D, D ensures a smooth starting operationof the vehicle on an up slope unless the vehicle displaces backwards for0.5 seconds after releasing the brake pedal BP. Also, provision of therelief valves RV, RV ensures a smooth starting operation of the vehicleon a slope, even if the driver has forcefully depressed the brake pedalBP than required. This is because brake fluid pressure within the wheelcylinders WC immediately decreases to a certain brake fluid pressurelevel (relief pressure) after the driver instantly or gradually releasesthe brake pedal BP.

Brake dragging will be caused if the solenoid valves SV(A), SV(B) arecontinuously kept in ON condition (closed condition) after the vehiclestarts moving on the slope. For this reason, the solenoid valves SV(A),SV(B) are preferably controlled so as to be OFF (open condition) whenthe driver initiates the starting operation. Specifically, the solenoidvalves SV(A), SV(B) are controlled to be OFF (open condition) when theaccelerator pedal is depressed in the case of automatic transmissionvehicles. Further, in view of fail-safe performance, the solenoid valvesSV(A), SV(B) may be controlled to be OFF (open condition) at a certainperiod of time (for example 2 to 3 seconds) after releasing the brakepedal BP. Depression and release of the brake pedal BP is detected bythe brake switch BSW. Alternatively, in order to prevent unintentionalbrake dragging, the solenoid valves SV(A), SV(B) may be controlled to beOFF (open condition) when the vehicle reaches to a certain speed (forexample 20 km/h).

(Stop/Start Operations on Down Slope)

When stopping the vehicle on a down slope, the driver depresses thebrake pedal BP. The control unit CU judges conditions including thevehicle being stopped, and turns the solenoid valves SV(A), SV(B) ON(closed condition) so as to retain brake fluid pressure within the wheelcylinder WC, thereby retaining braking force.

Same as in the case of an up slope, if the vehicle is on a steep downslope, displacement force derived from the vehicle's own weight may begreater than the retained braking force regardless of the solenoidvalves SV(A), SV(B) being in ON condition (closed condition). As aresult, the vehicle advances on the slope.

In order to start off the vehicle on the down slope, the driver thenreleases the brake pedal BP. On a down slope, the driver often startsthe vehicle without depressing the accelerator pedal. According to thebrake fluid pressure retaining unit RU, since the restrictions D, D areprovided, braking force gradually decreases after releasing the brakepedal BP. Braking force decreases even if the solenoid valves SV(A),SV(B) are ON condition (closed condition). Therefore, the driver canstart the vehicle without operating the accelerator pedal.

With the provision of the braking force retaining unit RU, the drivercan start the vehicle on a down slope without any difficulty. Further,the braking force retaining unit RU does not affect a smooth startoperation of the vehicle on a down slope and a flat place. Since thecontrol unit CU does not have to judge whether the vehicle is stopped ona slope or a flat surface, means for detecting slope is not necessary.

<Conditions for Retaining Brake Fluid Pressure>

Conditions that the braking force retaining unit RU retains brake fluidpressure will be described. As shown in FIG. 3A, brake fluid pressure isretained when all of the following four conditions are satisfied.

I) Brake switch BSW is ON.

II) Driving range is other than Neutral (N range), parking (P range) andReverse (R range).

III) Operation of the braking force retaining unit RU is permitted.

IV) Vehicle speed is 0 km/h.

When all the above conditions are satisfied, both solenoid valves SV(A),SV(B) are ON (closed condition), thereby retaining brake fluid pressurewithin the wheel cylinder WC. These conditions are judged by the controlunit CU.

I) The brake switch BSW has to be ON, otherwise no brake fluid pressureor few brake fluid pressure will be retained within the wheel cylindersWC. Also, the driver does not intend to apply brake on the vehicle whenthe brake switch BSW is OFF.

II) The driving range selected is other than N, P and R range (i.e. D orL range). This is for canceling unnecessary operation of the brakingforce retaining unit RU in N range or P range, and in R range, forpreventing the vehicle from unintentional backward displacement with theaid of driving force in the strong creep condition since the strongcreep condition is kept in the Reverse range.

Therefore, brake fluid pressure is retained while the driving range isin D range or L range.

III) Operation of the braking force retaining unit RU is permitted. Thisis for reminding the driver of sufficiently depressing the brake pedalBP before retaining brake fluid pressure so that unintentional backwarddisplacement of the vehicle on a slope can be prevented. Sincesufficient driving force is obtained in the strong creep condition suchthat the vehicle can stand still on a slope at an inclination angle of 5degrees, the driver often depresses the brake pedal BP insufficiently.Meanwhile, in the weak creep condition and the middle creep condition,driving force is not sufficient for stationarily retaining the vehicleon a slope having an inclination angle of 5 degrees. For this reason,the driver is reminded of forcefully depressing the brake pedal BP so asto retain sufficient brake fluid pressure for preventing backwarddisplacement on the slope. The control logic for permitting an operationof the braking force retaining unit RU will be described later.

IV) Vehicle speed is 0 km/h. This is because the driver can not select aposition for parking the vehicle if the solenoid valves SV(A), SV(B) areON (closed condition) during the vehicle is running. Meanwhile, sincethe vehicle stops while the vehicle speed is 0 km/h, braking force canbe retained without any troubles in the driving operation. “Vehiclespeed of 0 km/h” also includes a condition just before the vehiclestops.

[Conditions Required for Permitting Operation of the Braking ForceControl Unit]

With reference to FIG. 3B, conditions required for permitting anoperation of the braking force retaining unit RU will be described. Anoperation of the braking force retaining unit RU is permitted whiledriving force is either in the weak creep condition or in the middlecreep condition. In the weak creep condition and the middle creepcondition, driving force is not sufficient for stationarily retainingthe vehicle on a slope having an inclination angle of 5 degrees. Forthis reason, the driver is forced to depress the brake pedal BPsufficiently before retaining brake fluid pressure so as to retainsufficient brake fluid pressure (braking force) for preventing backwarddisplacement of the vehicle. Driving force in the weak creep conditionor in the middle creep condition is judged based on a hydraulic pressurecommand value to the linear solenoid valve of the CVT 3, where theengagement hydraulic pressure of the starting clutch is controlled.

[Conditions Required for Weak Creep Order]

Conditions for transmitting the weak creep order will be described. Asshown in FIG. 4A, the weak creep order (F_WCRP) is transmitted when anyof the following conditions I) and II) is satisfied. The conditions are:

I) Transmission is selected to N range or P range (N/P range).

II) [(1) Braking force retaining unit RU is normal; (2) Brake switch BSWis ON; (3) Advance range (D range/L range) is selected; and (4) Vehiclespeed is under or equal to 5 km/h] and further [(5) Vehicle speed afterswitching to the strong creep condition >5 km/h and vehicle speed>4km/h; or (6) Driving force is in the weak creep condition; or (7)Vehicle speed is 0 km/h, Driving force is in the middle creep condition,and a certain time has passed after switching to the middle creepcondition].

When one of the above conditions I) and II) is satisfied, the weak creeporder is transmitted and the driving force is switched to the weak creepcondition.

The above conditions are judged at the driving force control unit DCU.As mentioned above, the reason for switching driving force to the weakcreep condition is for reminding the driver of forcefully depressing thebrake pedal BP so as to prevent unintentional backward displacement ofthe vehicle as well as for improving fuel consumption of the vehicle.

I) When the transmission is selected to N range or P range, drivingforce is always switched to the weak creep condition. This is because ifthe transmission is selected from a non-driving range (N/P range) to adriving range (D/L/R range) and at the same time the accelerator pedalis quickly depressed, the driving force transmission capacity of thestarting clutch can be instantly increased, allowing a smooth startingoperation of the vehicle. In the weak creep condition, since pressureoil has been filled in an oil pressure chamber of the starting clutch,there is no clearance or play for the advance stroke of the pistonenforcing the clutch. Therefore, the driving force transmission capacityis instantly increased by increasing the pressure value of the pressureoil.

Driving force is switched to the weak creep condition when thetransmission is selected to N or P range. This is for previouslychanging the driving force transmission capacity of the starting clutchto the capacity at the weak creep condition. However, the driving forcefrom the engine 1 is not transmitted to the driving wheels 8, 8. This isdistinguished from the weak creep condition while the transmission isselected to D/L range.

In N/P range, connection between the engine 1 and the driving wheels 8,8is completely cut by a forward/reverse movement switching mechanismarranged in series with the starting clutch on a driving forcetransmission path. Since neither a transmission path for the forwardmovement nor a transmission path for the reverse movement is provided inN/P range, the driving force from the engine 1 is not transmitted to thedriving wheels 8, 8.

II) Conditions (1) to (4) are basic requirements for switching to theweak creep condition. Meanwhile, conditions (5) to (7) indicateconditions of the vehicle before switching to the weak creep condition.

(1) The braking force retaining unit RU is normal. Braking force is notretained if the braking force retaining unit RU is out of order. Sincesufficient driving force is not obtained in the weak creep condition,the vehicle will displace backwards on a slope. If the weak creep orderis transmitted and driving force is switched to the weak creep conditionregardless of abnormal conditions of the vehicle, eg the solenoid valvesSV(A), SV(B) are not switched to ON (closed condition), brake fluidpressure is not retained within the wheel cylinder WC when releasing thebrake pedal BP. Therefore, if the driver releases the brake pedal BPupon starting on the slope, braking force is suddenly lost and thevehicle displaces backwards. A smooth starting operation withoutunintentional backward displacement of the vehicle is therefore achievedby the strong creep condition.

(2) The brake switch BSW is ON. This is because the driver does notintend reduction of driving force.

(3) An advance range (D/L range) is selected. This is for improving fuelconsumption of the vehicle while an advance range is selected. When thepositioning switch PSW selects D range, driving force is switched to theweak creep condition notwithstanding the position (D mode/S mode) of themode switch MSW. However, in the R range, the driving force is notswitched to the weak creep condition. This is for facilitating asteering operation of the vehicle at a garage with the vehicle kept inthe strong creep condition.

(4) Vehicle speed is under or equal to 5 km/h. This is because drivingforce of the driving wheels 8, 8 is transmitted to the engine 1 or themotor 2 through the starting clutch of the CVT 3 so as to obtain enginebrake or carry out regenerative power generation by the motor 2.

(5) Vehicle speed after switching to the strong creep condition>5 km/hand the vehicle speed>4 km/h. This is because switching to the weakcreep condition is carried out merely by deceleration due to continuedbrake application. Since the difference of the driving force is greaterbetween the strong creep condition and the weak creep condition, thedriver may experience unintentional strong deceleration if the drivingforce is switched from the strong creep condition to the weak creepcondition by the depression of the brake pedal BP. Also, the vehicle maydisplace backwards in a moment if the vehicle stops on a slope. In suchcircumstance, it is preferable that the switching operation from thestrong creep condition to the weak creep condition is not carried out.To this end, once switched to the strong creep condition, the drivingforce is not changed to the weak creep condition until the throttle isOFF (depression of the accelerator pedal is released) over the vehiclespeed of 5 km/h and the driving force is switched to the strong creepcondition for driving. The vehicle speed may decrease to 5 km/h, afterbeing switched to the strong creep condition, without depression of thebrake pedal BP even if the vehicle once speeds up over 5 km/h and thenthe driving force is decreased (strong creep condition for driving). Forexample, when the vehicle moves on an up slope, the vehicle speed maydecrease without depression of the brake pedal BP. In such circumstance,since the brake switch BSW is OFF, the driving force is switched to thestrong creep condition when the vehicle speed decreases to 5 km/h. Inorder to cancel a successive switching operation from the strong creepcondition to the weak creep condition, a further condition, i.e.,vehicle speed>4 km/h is required. The switching operation to the weakcreep condition is not carried out unless the brake pedal BP isdepressed when the vehicle speed again decreases to 5 km/h. If the brakepedal BP is depressed (brake switch BSW [ON]) when the vehicle speedagain decreases to 5 Km/h, the driving force is switched from the strongcreep condition for driving to the weak creep condition. In other words,if the driving force is not changed to the weak creep condition when thevehicle speed again decreases to 5 km/h (vehicle speed=5 km/h), thestrong creep condition is retained as long as the vehicle speed is underor equal to 5 km/h.

(6) Driving force is in the weak creep condition. This is because onceswitched to the weak creep condition, the weak creep condition isretained regardless of the conditions (5) and (7). According to thecondition (5), driving force is switched to the weak creep conditionwhen the vehicle speed becomes 5 km/h. However, if the vehicle speed isless than 5 km/h, the condition (5) is not satisfied. The weak creepcondition is not retained merely by the condition (5) if the vehiclespeed is under 5 km/h. As a result, “driving force is in the weak creepcondition” is required so as to retain the weak creep condition underthe vehicle speed of 5 km/h. (7) Vehicle speed is 0 km/h, driving forceis in the middle creep condition, and a certain time has passed afterswitching to the middle creep condition. This is because deterioratedfuel consumption and vibration of the vehicle body, while the vehiclestops in the strong creep condition, are prevented with the drivingforce switched to the weak creep condition. The strong creep conditionis retained if driving force is not changed to the weak creep conditionwhen the vehicle speed again decreases to 5 km/h (vehicle speed=5 km/h)(based on the condition (5)), or if the vehicle speed under or equal to5 km/h is retained after switching to the strong creep condition by wayof releasing the brake pedal BP while being in the weak creep condition.However, if the vehicle stops in the strong creep condition with thebrake pedal depressed, fuel consumption is deteriorated and vibration ofthe vehicle remains. For this reason, when the vehicle completely stops(vehicle speed=0 km/h), driving force is switched to the middle creepcondition, in which driving force is between the strong creep conditionand the weak creep condition, and thereafter, if a certain time haspassed (300 msec in this embodiment), the driving force is furtherswitched to the weak creep condition. Since braking force due todepression of the brake pedal BP increases while driving force isstepwise reduced from the strong creep condition to the middle creepcondition and further to the weak creep condition, the momentarydisplacement amount of the vehicle on an up slope is restricted as smallas possible.

[Conditions Required for Strong Creep Condition for Driving]

Conditions required for strong creep condition for driving will bedescribed. Creep diving force is switched to the strong creep conditionfor driving after a strong creep order for driving (F_MSCRP) istransmitted. The strong creep order for driving (F_MSCRP) is transmittedwhen both of the following two conditions I) and II) are satisfied (FIG.4B).

I) Vehicle speed>5 km/h.

II) Throttle is OFF.

These conditions are judged at the driving force control unit DCU. Onereason for switching driving force to the strong creep condition fordriving is for preventing a strong deceleration of the vehicle beforestopping due to the switching operation from the strong creep conditionto the weak creep condition. Another reason is for preventing momentarybackward displacement of the vehicle on an up slope while the vehiclestops. Driving force is switched to the strong creep condition fordriving, which is weaker than the strong creep condition, in advance ofswitching to the weak creep condition.

I) vehicle speed>5 km/h. This is because the switching operation fromthe strong creep condition to the weak creep condition is carried out oncondition that the vehicle speed is once over 5 km/h after the strongcreep condition and then the vehicle speed becomes 5 km/h. This is alsofor discriminating between the strong creep condition under or equal tothe vehicle speed of 5 km/h and the strong creep condition for drivingover the vehicle speed of 5 km/h.

II) Accelerator pedal is not depressed (Throttle is OFF). Since thedriver does not intend further increment of the driving force, drivingforce may decrease without any problems.

[Conditions Required for Middle Creep Condition]

Conditions required for the middle creep condition will be described. Asshown in FIG. 4C, when the following three conditions I), II) and III)are satisfied, a middle creep order (F_MCRP) is transmitted.

I) Brake switch BSW is ON.

II) Advance range (D/L range) is selected.

III) Vehicle is fully stopped (vehicle speed=0 km/h).

These conditions are judged at the driving force control unit DCU. Thestrong creep condition is retained if driving force is not changed tothe weak creep condition when the vehicle speed again decreases to 5km/h (vehicle speed=5 km/h) or if the vehicle speed under or equal to 5km/h is retained after switching to the strong creep condition by way ofreleasing the brake pedal BP while being in the weak creep condition.However, if the vehicle remains stopping in the strong creep condition,fuel consumption is deteriorated and vibration of the vehicle continues.For this reason, the middle creep condition is required. As previouslymentioned, in order to prevent momentary backward displacement of thevehicle, which is due to switching from the strong creep condition tothe weak creep condition while the vehicle stops, driving force isswitched to the middle creep condition.

I) The brake switch BSW is ON. This is because the driver does notintend to reduce driving force when the brake pedal BP is not depressed.

II) Advance range (D/L range) is selected. It is necessary for switchingto the middle creep condition while an advance range is selected sincedriving force is switched to the weak creep condition while thepositioning switch is selected to D range or L range. Switching to themiddle creep condition is not necessary in N/P range since the weakcreep condition is selected as soon as the transmission is switched.Also, switching to the middle creep condition is not necessary in Rrange since the strong creep condition is retained in R range.

III) The vehicle is fully stopped (vehicle speed=0 km/h). Driving forceis switched to the weak creep condition in order to prevent deterioratedfuel consumption and vibration of the vehicle while the vehicle stops inthe strong creep condition. The middle creep condition is required as atransitional condition to the weak creep condition.

Judgement whether or not the driving force is in the weak creepcondition, strong creep condition for driving or the middle creepcondition is made based on the hydraulic pressure command value to thestarting clutch of the CVT 3.

[Conditions for Automatically Turning off the Engine]

For the purpose of further improvement of fuel consumption, the engine 1is automatically turned off while the vehicle stops. Conditions forautomatically turning off the engine 1 will be described. When all theconditions shown in FIG. 5 are satisfied, an engine stop order(F_ENGOFF) is transmitted and the engine 1 is automatically turned off.The automatic engine stop operation of the engine 1 is carried out bythe driving motor stopping unit. Therefore, the following automaticengine stop conditions are judged at the driving motor stopping unit.Specifically, the automatic engine stop conditions are judged at theFI/MG ECU 4 and the CVT ECU 6. When the FI/MG ECU 4 judges that all thefollowing conditions I) to VIII) are satisfied, the F_MGSTB becomes 1.When the CVT ECU 6 judges that all the following conditions IX) to XV)are satisfied, the F_CVTOK becomes 1.

I) Brake switch BSW is ON. This is for warning the driver. The driverplaces his foot on the brake pedal BP when the brake switch BSW is ON.Therefore, if the engine 1 is stopped and driving force is lost, thedriver can easily increase a brake pedal load before the vehicleunintentionally displaces backwards on a slope.

II) Water temperature of the engine is over a certain value. This isbecause the turn-on/turn-off operation of the engine 1 should be carriedout when the engine 1 is in stable conditions. In a cold area, if thewater temperature is low, the engine 1 may not restart.

III) Vehicle speed once reaches to 5 km/h after engine actuation. Thisis for facilitating a steering operation at a garage while the vehiclemoves in the creep running. The steering operation at a garage will betime-consuming if the engine 1 is turned off whenever the vehicle stopsfor changing steering directions.

IV) Positioning switch selects other than R range/D range (S mode)/Lrange, i.e., positioning switch selects N range/D range (D mode)/Prange. This is for the following reasons. A steering operation at agarage while selecting R rage or L range will be time-consuming if theengine 1 is turned off whenever the vehicle stops for changing steeringdirections. When the positioning switch PSW selects D range and the modeswitch MSW selects S mode, the driver is expecting a quick startoperation of the vehicle.

V) Capacity of the battery is over a certain value. If the remainingcapacity of the battery is not enough to restart the engine 1, the motorcannot actuate the engine 1 after turning off the engine.

VI) Electricity consumption is below a certain value. This is forsecuring sufficient electrical supply to loads.

VII) Load of the constant pressure chamber of the master power MP isover a certain value. This is because the smaller negative pressure inthe constant pressure chamber of the master power MP, the smalleramplification of the brake load when depressing the brake pedal BP,leading to deteriorated braking performance. Since negative pressure inthe constant pressure chamber is obtained from the intake pipe of theengine 1, negative pressure in the constant pressure chamber becomes farsmaller if the engine 1 is stopped at smaller negative pressures. Thisleads to reduced amplification of the brake load when the driverdepresses the brake pedal BP, and hence resulting in deterioratedbraking performance.

VIII) Accelerator pedal is not depressed (TH OFF). Since the driver doesnot intend further increment of driving force, the engine 1 may beautomatically turned off.

IX) All the automatic engine stop conditions at FI/MG ECU 4 aresatisfied. If all the engine stop conditions judged at the FI/MG ECU 4are not satisfied, it is not preferable to carry out the automaticengine stop operation.

X) Vehicle speed is 0 km/h. Driving force is not required when thevehicle stops.

XI) Ratio of the CVT is low. This is because a smooth starting operationof the vehicle is not carried out unless the ratio of the CVT (pulleyratio) is low.

XII) Oil temperature of the CVT is over a certain value. If the oiltemperature of the CVT 3 is low, start-up for hydraulic pressure of thestarting clutch will cause a delay. Therefore, the required time fromthe engine actuation to the strong creep condition is extended, and thevehicle will displace backwards on a slope.

XIII) Accelerator pedal is not depressed (TH OFF). Since the driver doesnot intend further increment of driving force, the engine 1 may beautomatically turned off.

XIV) Braking force retaining unit RU is normal. Since brake fluidpressure may not be retained if the braking force retaining unit RU isout of order, the strong creep condition is kept for preventing thevehicle from unintentional backward displacement.

XV) [(1) Brake fluid pressure is retained (solenoid valves SV(A), SV(B)are ON (closed condition), and Brake switch BSW is ON] or [(2)Positioning switch PSW selects N range/P range]. This is for thefollowing reasons:

(1) As long as brake fluid pressure that is braking force is retained,the vehicle does not displace backwards on a slope even if the engine 1is automatically turned off and driving force is lost. Further, when thebrake switch BSW is ON, the driver places his foot on the brake pedalBP. Therefore, if the engine 1 is stopped and the driving force is lost,the driver can easily increase a brake pedal load before the vehicleunintentionally displaces backwards on a slope.

(2) If the vehicle stops with the positioning switch PSW selecting Prange or N range, the driver intends to pull up the vehicle.Therefore,the engine 1 may be automatically turned off. In thiscondition, the engine 1 is automatically turned off even if the brakingforce retaining unit RU is not actuated and no braking force isretained.

<Conditions for Releasing Retained Brake Fluid Pressure>

Conditions that the braking force retaining unit RU releases retainedbrake fluid pressure will be described. As shown in FIG. 6A, retainedbrake fluid pressure is released when any of the following conditions issatisfied:

I) Positioning switch PSW selects N range/P range and Brake switch BSWis OFF.

II) A certain delay time has passed after brake switch BSW becomes OFF.

III) Creep driving force has risen and Brake switch BSW is OFF.

IV) Vehicle speed is over 20 km/h.

When any of the above conditions is satisfied, the two solenoid valvesSV(A), SV(B) are OFF (open condition) for releasing brake fluid pressureretained within the wheel cylinders WC. The above conditions are judgedat the control unit CU.

I) The positioning switch PSW selects N range/P range and the brakeswitch BSW is OFF. This is for eliminating unnecessary operation of thebraking force retaining unit RU.

II) A certain delay time has passed after brake switch BSW becomes OFF.It is not preferable as a fail-safe action that brake fluid pressure ispermanently retained after releasing the brake pedal BP and brakedragging occurs. In the preferred embodiment, the delay time is about 2seconds after releasing the brake pedal BP, i.e. after the brake switchBSW is OFF.

III) Creep driving force has risen and Brake switch BSW is OFF. This isfor canceling retention of unintentional and unnecessary braking forcewhile sufficient driving force occurs against a slope. The reason forreleasing retained brake fluid pressure when creep driving force isrisen is that there is no need to retain brake fluid pressure within thewheel cylinder WC so as prevent backward displacement of the vehiclesince when creep driving force is risen, sufficient driving force isobtained for stationary retaining the vehicle on the slope even if thebraking force retaining unit RU is not actuated. Further, whendepression of the brake pedal BP is released (brake switch BSW is OFF),the driver does not require braking force.

IV) Vehicle speed is over 20 km/h. This is for eliminating unnecessarybrake dragging as a fail-safe action.

[Requirement for Creep Rising Condition]

Requirement for a creep rising condition will be described. As shown inFIG. 6B, when any of the following conditions I) and II) is satisfied,it is considered that the creep diving force has risen.

I) Hydraulic pressure command value of the starting clutch at the CVT 3is over a certain value.

II) A certain time has passed after the engine 1 is automatically turnedoff and then restarted.

These two conditions are judged at the driving force control unit DCU.In the creep rising condition, driving force has been increased to suchan extent that backward displacement of the vehicle on the slope isprevented, even if operation of the braking force retaining unit RU isdisengaged and no braking force is retained. Specifically, in the creeprising condition, driving force has been increased to such an extentthat backward displacement of the vehicle on the slope is prevented,taking into consideration the inertial force and the rolling resistance(increasing driving force) of the vehicle. The creep rising conditionalso includes a condition allowing slight backward displacement of thevehicle, as long as the increasing driving force minimize the backwarddisplacement of the vehicle.

I) When the hydraulic pressure command value of the starting clutch atthe CVT 3 is over a certain value, driving force has been increased tosuch an extent that backward displacement of the vehicle is preventedeven if retained brake fluid pressure is released. “The hydraulicpressure command value over a certain value” indicates that thehydraulic pressure command value—it is transmitted to the linearsolenoid valve, which controls the hydraulic pressure for the engagingforce of the starting clutch—has been increasing to a half value betweenthe weak creep condition and the strong creep condition, in the processswitching from the weak creep condition to the strong creep condition.

II) A certain time has passed after the engine 1 is automatically turnedoff and then restarted. It is considered that if retention of brakefluid pressure is released, sufficient driving force has been increasedagainst the slope. Time-counting is initiated when the engine 1 isautomatically restarted and supply of the pressure oil to the startingclutch is started. Hydraulic oil has been discharged from the oilpressure chamber of the starting clutch at the CVT 3 while the engine 1is turned off. Therefore, a clearance or play exists for the advancestroke of the piston enforcing the clutch when the engine 1 is actuatedand supply of the pressure oil is initiated. For this reason, thehydraulic pressure command value to the linear solenoid valve of thestarting clutch does not correspond to the actual hydraulic pressurevalue (driving force transmission capacity). When increasing the drivingforce from the engine stop condition, it is impossible to judge thecreep rising condition based on the hydraulic pressure command value ofthe starting clutch. As a result, the creep rising condition is judgedwhen a timer counts a certain period time after the supply of thepressure oil to the starting clutch is initiated.

[Conditions Required for Strong Creep Order]

Conditions for the strong creep order will be described. The strongcreep order (F_SCRP) is transmitted when any of the following twoconditions shown in FIGS. 7A and 7B is satisfied. The first conditionrequired for the strong creep order is that either I) or II) issatisfied (FIG. 7A).

I) [(1) Brake switch is OFF or Throttle is ON, and Advance range (D/Lrage) is selected] or [(2) Reverse (R) range is selected] and (3)Vehicle speed is under or equal to 5 km/h.

II) Backward displacement of the vehicle is detected.

Meanwhile, the second condition required for the strong creep order isthat either III) or IV) is satisfied (FIG. 7B).

III) [(1) Brake switch is OFF or Throttle is ON, and Advance range (D/Lrange) is selected] or [(2) Reverse (R) range is selected] and (3)Vehicle speed is under or equal to 5 km/h.

IV) Vehicle speed pulse is input and Vehicle is fully stopped before theinput of vehicle speed pulse.

In the first and the second conditions required for the strong creeporder, I) and III) are identical, while II) and IV) are different.Therefore, explanation of the condition III) is omitted. Theseconditions I) to IV) are judged at the driving force control unit DCU.

(1) Brake switch is OFF or throttle is ON, and advance range (D/L rage)is selected. Since the driver initiates a starting operation, drivingforce is changed to the strong creep condition. The driver has anintention to start the vehicle since the positioning switch PSW isselected to D range or L range and further depression of the brake pedalBP is released or instead the accelerator pedal is depressed. Therefore,the driving force is switched from the weak creep condition to thestrong creep condition.

With the depression of the accelerator pedal, the driving forcetransmission capacity increases, even after reaching to the greaterdriving force transmission capacity (strong creep condition), to acapacity allowing to transmit all the driving force generated at thedriving motor (condition greater than the greater driving forcetransmission capacity). However, the flag showing the strong creepcondition (F_SCRPON ) is kept until another flag rises.

(2) Reverse (R)range is selected. This is for ensuring a smooth creepdriving in R range. When the positioning switch PSW is selected to Rrange, the driver expects a steering operation at a garage with thedriving force switched to the strong creep condition. Therefore, thedriving force is switched from the weak creep condition to the strongcreep condition.

(3) The vehicle speed under or equal to 5 km/h. This is because thestrong creep condition for driving at a vehicle speed over 5 km/h can bedistinguished from the strong creep condition at a vehicle speed underor equal to 5 km/h.

II) Backward displacement of the vehicle is detected. When the vehiclestarts to displace backwards on a steep slope with the backwarddisplacement force derived from the vehicle's own weight being greaterthan the braking force, the driving force in the strong creep conditionprevents the backward displacement of the vehicle. When the vehiclestops on an up slope, the total amount of the driving force in the weakcreep condition (the driving force is zero if the engine 1 isautomatically turned off) and the braking force resists the backwarddisplacement force of the vehicle. However, since the greaterinclination angle of the slope, the greater backward displacement force,the vehicle starts to displace backwards on the steep slope with thebackward displacement force being greater than the total amount of thedriving force in the weak creep condition and the braking force. Forthis reason, when backward displacement of the vehicle is detected, thedriving force is switched from the weak creep condition (engine stopcondition when the engine 1 is turned off) to the strong creep conditionin any circumstances so as to generate sufficient driving force againstthe slope.

With reference to FIG. 9, means for detecting backward displacement ofthe vehicle will be described. For example, helical gears HG(A), HG(B)are provided at a downstream of the starting clutch of the CVT 3. Thehelical gears HG(A), HG(B) may be provided at any positions as long asthey are rotatable with the tires. As shown in FIG. 9A, gear tooth ofthe helical gears HG(A), HG(B) are positioned in helical and diagonalrelation around the periphery of the gear. The phase of the gear toothshifts with the rotation of the helical gears HG(A), HG(B) in {circlearound (1)} and {circle around (2)} directions. To this end,electromagnetic pick-ups P(A), P(B) are provided on the respectivehelical gears HG(A), HG(B) so as to align in the same axis AX of thehelical gears. The electromagnetic pick-ups P(A), P(B) detect the frontends of the gear tooth. Direction of the rotation is obtained from thepulse phase difference based on the two pulses detected at theelectromagnetic pick-ups P(A), P(B). As best seen in FIG. 9B, when thehelical gears HG(A), HG(B) rotate to the {circle around (1)} direction,the pulse detected at the electromagnetic pick-up P(B) shifts back fromthat detected at the electromagnetic pick-up P(A). In other words, thefront end of the gear teeth of the helical gear HG(A) is detected beforethat of the gear teeth of the helical gear HG(B). Meanwhile, when thehelical gears HG(A), HG(B) rotate to the {circle around (2)} direction,the pulse detected at the electromagnetic pick-up P(B) shifts forward tothat detected at the electromagnetic pick-up P(A) (FIG. 9C). In otherwords, the front end of the gear teeth of the helical gear HG(A) isdetected after that of the gear teeth of the helical gear HG(B).Direction of the rotation is therefore detected by the pulse phasedifference. Supporting that the rotation in the {circle around (1)}direction indicates backward displacement of the vehicle, backwarddisplacement is detected by the relative positions of the two pulsesobtained from the electromagnetic pick-ups P(A), P(B) mentioned above.As long as having a phase difference, any known gears other than helicalgears HG(A), HG(B) may be employed.

IV) A vehicle speed pulse is input and the vehicle is fully stoppedbefore the input of the vehicle speed pulse. This is for the followingreason. When the vehicle displaces from the fully stopped position,backward displacement (possible backward displacement) of the vehicle isdetected and then the driving force is switched to the strong creepcondition so as to keep the vehicle against the slope. Althoughdisplacement of the vehicle is detected, judgement is not carried out tospecify the direction as to whether the vehicle moves forward orbackwards. When the vehicle stops on an up slope, the total amount ofthe driving force in the weak creep condition (the driving force is zeroif the engine 1 is automatically turned off) and the braking forceresists the backward displacement force of the vehicle. However, sincethe greater inclination angle of the slope, the greater backwarddisplacement force, the vehicle starts to displace forward (on a downslope) or backwards (on an up slope) with the displacement force derivedfrom the vehicle's own weight being greater than the total amount of thedriving force in the weak creep condition and the braking force. Forthis reason, when forward or backward displacement (i.e. displacement)of the vehicle is detected, the driving force is switched from the weakcreep condition (engine stop condition when the engine 1 is turned off)to the strong creep condition so as to generate sufficient driving forceagainst the slope. For the purpose of the detection that the vehiclecompletely stops, the vehicle speed pulse of zero is detected before avehicle speed pulse is input. Displacement of the vehicle is detectedeven from one vehicle speed pulse input. The driving force may beswitched to the strong creep condition even if the vehicle displaces tothe same direction as the driver's intention.

[Conditions for Automatically Turning On the Engine]

After automatically turning off the engine 1, the engine 1 isautomatically restarted in the following conditions. When any of thefollowing conditions shown in FIGS. 8A and 8B is satisfied, theautomatic engine actuation order (F_ENGON) is transmitted and the engine1 is automatically actuated. The automatic engine actuation is carriedout by the driving motor stopping unit. Therefore, the followingautomatic engine actuation conditions are judged at the driving motorstopping unit. Specifically, the automatic engine actuation conditionsare judged at the FI/MG ECU 4 and the CVT ECU 6. When the FI/MG ECU 4judges that any of the following conditions I) to VI) is satisfied, theF_MGSTB becomes 0. When the CVT ECU 6 judges that any of the followingconditions VII) to XI) [or VII) to X) and XII)] is satisfied, theF_CVTOK becomes 0. When at least one of the F_MGSTB and the F_CVTOKbecomes 0, the engine actuation order (F ENGON) is transmitted. Thefirst condition required for the automatic engine actuation order (shownin FIG. 8A) is the same as the second condition shown in FIG. 8B, exceptfor the conditions XI) and XII) which are judged by the CVT ECU 6.Therefore, explanation refers merely to the condition XII) as to thesecond condition required for the automatic engine actuation order.

I) Depression of the brake pedal BP is released (Brake switch BSW isOFF). This is because judgement of the starting operation is carried outwhen the driver releases the brake pedal BP. When the driver releasesthe brake pedal BP in D range/D mode, it is considered that the driverinitiates the starting operation. Therefore, the engine 1 isautomatically actuated. Meanwhile, the driver releases the brake pedalBP in P range or N range so as to pull off and leave from the vehicle.In this circumstance, the engine 1 is automatically actuated in order toremind the driver not to leave from the vehicle without turning off theignition switch.

II) R range/D range (S mode)/L range is selected. This is because thedriver intends to start the vehicle quickly if the transmission isselected to R range/D range (S mode) /L range after the engine 1 isturned off. Therefore, when the engine 1 is turned off with thetransmission selected other than R range/D range (S mode)/L range andthereafter switched to R range/D range (S mode)/L range, the engine 1 isautomatically actuated.

III) Remaining capacity of the battery is below a certain value. This isbecause the engine 1 is not automatically actuated if the remainingcapacity of the battery is not enough. The engine 1 is not turned offunless the remaining capacity of the battery is over a certain value.However, capacity of the battery may lower after the engine 1 isautomatically turned off. In this case, the engine 1 is automaticallyactuated for the purpose of battery charge. The certain value is set tobe higher than the critical battery capacity, below which the engine 1is not actuated.

IV) Electricity consumption is over a certain value. While electricityconsumers such as lights are working on, capacity of the batterydecreases quickly. As a result, the engine 1 will not be restarted. Forthis reason, irrespective of the remaining capacity of the battery, theengine 1 is automatically actuated when the electricity consumption isover a certain value.

V) Negative pressure of the master power MP is below a certain value.The lower the negative pressure at the master power MP, the less brakingforce is obtained. Therefore, the engine 1 is restarted for securingsufficient braking force.

VI) Accelerator pedal is depressed (TH ON). This is because the driveris expecting driving force by the engine 1. Therefore, the engine 1 isautomatically actuated when the accelerator pedal is depressed.

VII) Engine 1 is actuated while the automatic engine stop conditions atFI/MGECU 4 are not satisfied. This is because the engine 1 has to beactuated since at least one of the automatic engine actuation conditionsis satisfied.

VIII) Accelerator pedal is depressed (throttle is ON). This is becausethe driver is expecting driving force by the engine 1. Therefore, theengine 1 is automatically actuated when the accelerator pedal isdepressed.

IX) Depression of the brake pedal BP is released (Brake switch BSW isOFF). This is because judgement of the starting operation is carried outwhen the driver releases the brake pedal BP. When the driver releasesthe brake pedal BP in D range/D mode, it is considered that the driverinitiates the starting operation. Therefore, the engine 1 isautomatically actuated.

X) Braking force retaining unit RU is out of order. When the brakingforce retaining unit RU is out of order and braking force is notretained, the vehicle displaces backwards (forward) on a slope with theautomatic engine stop operation. Therefore, when the solenoid valvesSV(A), SV(B) of the braking force retaining unit RU are out of order,the engine 1 is automatically actuated and the vehicle is kept in thestrong creep condition. If failure is detected in the braking forceretaining unit RU after turning off the engine 1, the engine 1 isimmediately actuated such that driving force of the vehicle is kept inthe strong creep This is because braking force may not be retained afterreleasing the brake pedal BP upon starting the vehicle. In other words,it is the strong creep condition that prevents the vehicle fromunintentional backward displacement and facilitates a smooth startingoperation of the vehicle.

XI) Backward displacement of the vehicle is detected. When the vehiclestarts to displace backwards on a steep slope with the backwarddisplacement force derived from the vehicle's own weight being greaterthan braking force, the vehicle is prevented from backward displacementby driving force of the engine 1. When the vehicle stops on an up slope,braking force resists the backward displacement force of the vehicle.However, since the greater inclination angle of the slope, the greaterbackward displacement force, the vehicle starts to displace backwards onthe steep slope with the backward displacement force being greater thanthe braking force. For this reason, when backward displacement of thevehicle is detected, the driving force is switched from the engine stopcondition to the strong creep condition in any circumstances so as togenerate sufficient driving force against the slope. Since the way ofdetecting backward displacement of the vehicle has been referred in[Conditions required for Strong Creep Order], further explanation willbe omitted.

XII) A vehicle speed pulse is input and the vehicle is fully stoppedbefore the input of the vehicle speed pulse. This is for the followingreason. When the vehicle displaces from the fully stopped position,backward displacement (possible backward displacement) of the vehicle isdetected and then the engine 1 is automatically actuated so as togenerate driving force against the slope. Although displacement of thevehicle is detected, judgement is not carried out to specify thedirection as to whether the vehicle moves forward or backwards. When thevehicle stops on an up slope with the engine 1 turned off, merelybraking force resists the backward displacement force of the vehicle.However, since the greater inclination angle of the slope, the greaterbackward displacement force, the vehicle starts to displace forward (ona down slope) or backwards (on an up slope) with the displacement forcederived from the vehicle's own weight being greater than the brakingforce. For this reason, when forward or backward displacement (i.e.displacement) of the vehicle is detected, the engine 1 is automaticallyactuated so as to generate sufficient driving force in the strong creepcondition. For the purpose of detection that the vehicle completelystops, the vehicle speed pulse of zero is detected before a vehiclespeed pulse is input. Displacement of the vehicle is detected even fromone vehicle speed pulse input.

<Time Chart for Control with Automatic Engine Stop Operation>

With reference to FIG. 10, the way of control will be described for avehicle having the above system configuration. The vehicle is operatedin the order of braking, stopping and starting. In this control, drivingforce is switched from the strong creep condition for driving to theweak creep condition by the driving force control unit DCU, and theengine 1 is turned off by the driving motor stopping unit. The vehicleis supposed to stop on an up slope. The positioning switch PSW and themode switch MSW of the vehicle are not changed from D range/D mode. Thebraking force retaining unit RU is provided with a relief valve RV.Further, the vehicle is provided with means for detecting backwarddisplacement.

In the time chart of FIG. 10(a), the processes of increasing anddecreasing driving force and braking force of the vehicle are shown intime sequence order. Here, a thick line indicates driving force and athin line indicates braking force. In the time chart of FIG. 10(b),ON/OFF (close/open) of the solenoid valves SV(A), SV(B) is shown.

The driving force control unit DCU transmits a strong creep order fordriving (F_MSCRP) when the driver releases the accelerator pedal(throttle is OFF) during the vehicle is running (vehicle speed>5 km/h).Driving force is then switched to the strong creep condition for driving(F_MSCRPON), which is less than the strong creep condition (F_SCRPON).

If the driver releases the accelerator pedal and depresses the brakepedal BP (brake switch BSW is ON), braking force increases. When thevehicle speed falls to 5 km/h with continued braking application, thedriving force control unit DCU transmits a weak creep order (F_WCRP) anddriving force is switched to the weak creep condition (F_WCRPON). Whendo so, since the driving force decreases to the weak creep conditionthrough the strong creep condition for driving, the driver does notexperience a strong deceleration.

When the vehicle speed falls to 0 km/h, the braking force retaining unitRU turns the solenoid valves SV(A), SV(B) ON (closed condition) so as toretain braking force. Further, the driving motor stopping unitautomatically turns off the engine 1 (F_ENGOFF) and the driving force islost. When do so, the solenoid valves SV(A), SN(B) are ON (closedcondition) and brake fluid pressure is retained within the wheelcylinders WC. Since the engine 1 is turned off after the weak creepcondition, the driver forcefully depresses the brake pedal BP to such anextent that the vehicle is prevented from backward displacement on aslope. For this reason, even if the engine 1 is automatically turnedoff, the vehicle does not displaces backwards on the slope (backwarddisplacement restriction force). The vehicle receives displacement forcedue to its own weight on an up slope. However, since the retainedbraking force is greater than the displacement force due to thevehicle's own weight, the vehicle does not displace backwards. Theengine 1 is automatically turned off for the purposes of improved fuelconsumption as well as restriction of exhaust gas.

The driver then loosens depression of the brake pedal BP in order tostand by for restarting the vehicle. If the diver depresses the brakepedal BP more than a preset pressure of the relief valve RV (reliefpressure), the relief valve RV actuates upon the driver loosening thebrake pedal BP and braking force immediately decreases to the reliefpressure. Provision of the relief valve RV ensures a smooth startingoperation of the vehicle on a slope even if the driver depresses thebrake pedal BP more than required. Loosening depression of the brakepedal BP does not mean to completely release depression of the brakepedal BP, but to depress the brake pedal BP to a certain extent.Therefore, the brake switch BSW continuously keeps ON when depression ofthe brake pedal BP is loosened.

When the driver loosens depression of the brake pedal BP and brake fluidpressure falls below the relief pressure, brake fluid pressure withinthe wheel cylinders WC gradually decreases by the restriction D of thebraking force retaining unit RU. Braking force decreases accordingly. Inthe end, the retained braking force decreases lower than thedisplacement force derived from the vehicle's own weight. As a result,the vehicle starts to displace backwards. The vehicle then detectsbackward displacement and transmits an engine automatic actuation order(F_ENGON). After a time lag derived from a delay of signal communicationand mechanisms, the engine 1 is automatically actuated and supply ofpressure oil to the starting clutch at the CVT 3 is initiated (SC [ON]).

Hydraulic oil has been discharged from the oil pressure chamber of thestarting clutch at the CVT 3 while the engine 1 is turned off.Therefore, when the engine 1 is actuated and supply of pressure oil tothe starting clutch is initiated, driving force rises due to resistanceof a piston enforcing the clutch. Further, a clearance or play existsfor the advance stroke of the piston while the engine 1 is turned offsince hydraulic oil has been discharged. Therefore, the hydraulicpressure command value to the starting clutch does not correspond to theactual hydraulic pressure value (driving force transmission capacity)hence the driving force transmission capacity of the starting clutchincreases little by little until the oil pressure chamber is filled withhydraulic oil. As a result, driving force gradually increases, and whenthe oil pressure chamber is filled with hydraulic oil, the driving forcethen increases to the strong creep condition (F_SCRPON) in accordancewith the hydraulic pressure command value. During the increment to thestrong creep condition (F_SCRPON), the solenoid valves SV(A), SV(B) arein ON condition (closed condition) and brake fluid flows to the mastercylinder MC only through the narrow restriction D such that brakingforce gradually decreases. Braking force corresponds to the brake pedaldepression after loosening thereof.

As mentioned above, when backward displacement of the vehicle isdetected, the automatic engine actuation operation is initiatedimmediately, thereby increasing driving force. As a result, the totalamount of the increased driving force and the retained braking forcebecomes greater than the displacement force derived from the vehicle'sown weight, preventing backward displacement of the vehicle.

As shown in FIG. 10(a), a phantom line is extending downwardly from“Relief pressure” on the line indicating braking force. The phantom lineindicates a case when brake fluid pressure is not retained. In thiscase, reduction of braking force takes place immediately afterdecreasing brake pedal load applied by the driver.

<Time Chart for Control Without Automatic Engine Stop Operation>

With reference to FIG. 11, the way of control will be described for avehicle having the above system configuration. The vehicle is operatedin the order of braking, stopping and starting. In this control, drivingforce is switched from the strong creep condition for driving to theweak creep condition by the driving force control unit DCU. However, theengine 1 is not automatically turned off. The vehicle is supposed tostop on an up slope. The positioning switch PSW and the mode switch MSWof the vehicle are not changed from D range/D mode. The braking forceretaining unit RU is provided with a relief valve RV. The vehicle isprovided with means for detecting backward displacement.

In the time chart of FIG. 11(a), the processes of increasing anddecreasing driving force and braking force of the vehicle are shown intime sequence order. Here, a thick line indicates driving force and athin line indicates braking force. In the time chart of FIG. 11(b),ON/OFF (close/open) of the solenoid valves SV(A), SV(B) is shown.

Since the processes before driving force is switched to the weak creepcondition are the same as those described in <Time Chart for Controlwith Automatic Engine Stop Operation>, further explanation is omitted.

When the vehicle speed falls to 0 km/h, the braking force retaining unitRU turns the solenoid valves SV(A), SV(B) ON (closed condition) so as toretain braking force. However, the engine 1 is not automatically turnedoff since the automatic engine stop conditions are not satisfied or thevehicle is not provided with the driving motor stopping unit. With thesolenoid valves SV(A), SV(B) switched to ON (closed condition), brakefluid pressure is retained within the wheel clinder WC. Since thevehicle stops after the weak creep condition, the driver forcefullydepresses the brake pedal BP to such an extent that the vehicle isprevented from backward displacement on a slope. For this reason, evenif driving force is in the weak creep condition, the vehicle does notdisplaces backwards on the slope (backward displacement restrictionforce). When the vehicle stops on an up slope, the vehicle weightaffects to displace the vehicle. However, the vehicle does not displacebackwards since the total amount of the driving force in the weak creepcondition and the braking force to be retained is greater than thebackward displacement force due to the vehicle weight.

The driver then loosens depression of the brake pedal BP in order tostand by for restarting the vehicle. If the diver depresses the brakepedal BP more than a preset pressure of the relief valve RV (reliefpressure), the relief valve RV actuates upon the driver loosening thebrake pedal BP and braking force immediately decreases to the reliefpressure. Provision of the relief valve RV ensures a smooth startingoperation of the vehicle on a slope even if the driver depresses thebrake pedal BP more than required.

When the driver loosens depression of the brake pedal BP and brake fluidpressure falls below the relief pressure, brake fluid pressure withinthe wheel cylinders WC gradually decreases by the restriction D of thebraking force retaining unit RU. Braking force decreases accordingly. Inthe end, the total amount of driving force in the weak creep conditionand the retained braking force decreases lower than the displacementforce derived from the vehicle's own weight. As a result, the vehiclestarts to displace backwards. The vehicle then detects backwarddisplacement and transmits a strong creep order (F_SCRP).

In the weak creep condition, the oil pressure chamber of the startingclutch has been filled with hydraulic oil. Since no clearance or playexists for the advance stroke of the piston, the hydraulic pressurecommand value to the starting clutch corresponds to the actual hydraulicpressure value (driving force transmission capacity). As a result,driving force increases to the strong creep condition (F_SCRPON) inaccordance with the hydraulic pressure command value. During theincrement to the strong creep condition (F_SCRPON), the solenoid valvesSV(A), SV(B) are in ON condition (closed condition) and brake fluidflows to the master cylinder MC only through the narrow restriction Dsuch that braking force gradually decreases. Braking force correspondsto the brake pedal depression after loosening thereof.

As mentioned above, when backward displacement of the vehicle isdetected, the strong creep order is transmitted immediately, therebyincreasing driving force. As a result, the total amount of the increaseddriving force and the retained braking force becomes greater than thedisplacement force derived from the vehicle's own weight, preventingbackward displacement of the vehicle.

As shown in FIG. 11(a), a phantom line is extending downwardly from“Relief pressure” on the line indicating braking force. The phantom lineindicates a case when the brake fluid pressure is not retained. In thiscase, reduction of braking force takes place immediately afterdecreasing brake pedal load applied by the driver.

In the vehicle with a braking force retaining unit according to thepresent invention, backward displacement of the vehicle can be preventedsince driving force is immediately increased when displacement forcederived from the vehicle's own weight becomes greater than the retainedbraking force and backward displacement (or displacement) of the vehicleoccurs. Further, there is no need to employ a pressuring device (such asa hydraulic pump in the case of hydraulically operable braking device)since, in order to restrict backward displacement of the vehicle,driving force is increased without increasing braking force.

While the present invention has been described by way of a specificexample, it is to be understood that changes and variations may be madewithout departing from the spirit or scope of the following claims.

For example, the braking force retaining unit has been described to beconstructed from means for acting on brake fluid pressure. However, aslong as acting on braking force, other means may be employed.

Also, helical gears and electromagnetic pick-ups are employed as meansfor detecting backward displacement of the vehicle. However, other meansmay be employed as long as displacement (or backward displacement) ofthe vehicle can be detected.

What is claimed is:
 1. A vehicle with a braking force retaining unit,which retains a braking force after releasing a brake pedal, wherein thevehicle further comprises means for automatically increasing a drivingforce if displacement of the vehicle is detected while the braking forceretaining unit is in operation.
 2. A vehicle with a braking forceretaining unit, which retains a braking force after releasing a brakepedal, wherein a driving force of the vehicle is automatically increasedif displacement of the vehicle is detected while the braking forceretaining unit is in operation.
 3. The vehicle according to claim 1,wherein the means for automatically increasing a driving force includesa driving force control unit which has a continuously variabletransmission electronic control unit.
 4. The vehicle according to claim3, wherein the driving force control unit facilitates the increase ofthe driving force to a strong creep condition upon detection of thedisplacement of the vehicle while the braking force retaining unit isactivated.